From: MonkeeSage <MonkeeSage@...>
Date: 2006-10-04T23:50:18+09:00
Subject: Re: Equvialent of Python slices?

Hi Peter,

In ruby you can extend built-in objects, so you can get the same
(general) functionality by adding a method to Enumerable (which is
mixed-in to String and Array [and Hash, too, but step is not meaningful
for that class]):

module Enumerable
  def step(*args)
    start, stop = 0, -1
    case args.size
      when 1
        step  = args[0]
      when 2
        start = args[0]
        step  = args[1]
      when 3
        start = args[0]
        stop  = args[1]
        step  = args[2]
      else
        raise(ArgumentError,
              "`step': wrong number of " +
              "arguments (#{args.size} for 3)",
              caller)
    end
    if self.is_a?(Hash)
      raise(NoMethodError,
            "`step': not implemented for Hash",
            caller)
    end
    seq = self[start..stop]
    pos = 0
    if self.is_a?(Array)
      out = seq[0,1]
    elsif self.is_a?(String)
      out = seq[0,1].to_s
    end
    while pos < seq.size
      val = seq[(pos += step)]
      out << val unless val.nil?
    end
    out
  end
end

# start at index 2 and step by 2...
p "abcdefg".step(2, 2) # => "ceg"
p %w{a b c d e f g}.step(2, 2) # => ["c", "e", "g"]

# this breaks
{'a'=>'b'}.step(2) # => -:60: `step': not implemented for Hash
(NoMethodError)

Regards,
Jordan